Discovered: Cosmic Raysfrom a Mysterious, Nearby Object

Nov.
19, 2008: An international team of researchers has
discovered a puzzling surplus of high-energy electrons bombarding
Earth from space. The source of these cosmic rays is unknown,
but it must be close to the solar system and it could be made
of dark matter. Their results are being reported in the Nov.
20th issue of the journal Nature.

"This
is a big discovery," says co-author John Wefel of Louisiana
State University. "It's the first time we've seen a discrete
source of accelerated cosmic rays standing out from the general
galactic background."

Galactic
cosmic rays are subatomic particles accelerated to almost
light speed by distant supernova explosions and other violent
events. They swarm through the Milky Way, forming a haze of
high energy particles that enter the solar system from all
directions. Cosmic rays consist mostly of protons and heavier
atomic nuclei with a dash of electrons and photons spicing
the mix.

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To
study the most powerful and interesting cosmic rays, Wefel
and colleagues have spent the last eight years flying a series
of balloons through the stratosphere over Antarctica. Each
time the payload was a NASA-funded cosmic ray detector named
ATIC, short for Advanced Thin Ionization Calorimeter. The
team expected ATIC to tally the usual mix of particles, mainly
protons and ions, but the calorimeter found something extra:
an abundance of high-energy electrons.

Wefel
likens it to driving down a freeway among family sedans, mini-vans
and trucks—when suddenly a bunch of Lamborghinis bursts through
the normal traffic. "You don't expect to see so many
race cars on the road—or so many high-energy electrons in
the mix of cosmic rays." During five weeks of ballooning
in 2000 and 2003, ATIC counted 70 excess electrons in the
energy range 300-800 GeV. ("Excess" means over and
above the usual number expected from the galactic background.)
Seventy electrons may not sound like a great number, but like
seventy Lamborghinis on the freeway, it's a significant surplus.

Above:
ATIC high-energy electron counts. The triangular curve fitted
to the data comes from a model of dark-matter annihilation
featuring a Kaluza-Klein particle of mass near 620 GeV. Details
may be found in the Nov. 20, 2008, edition of Nature: "An
excess of cosmic ray electrons at energies of 300-800 Gev,"
by J. Chang et al. [

"The
source of these exotic electrons must be relatively close
to the solar system—no more than a kiloparsec away,"
says co-author Jim Adams of the NASA Marshall Space Flight
Center.

Why
must the source be nearby? Adams explains: "High-energy
electrons lose energy rapidly as they fly through the galaxy.
They give up energy in two main ways: (1) when they collide
with lower-energy photons, a process called inverse Compton
scattering, and (2) when they radiate away some of their energy
by spiraling through the galaxy's magnetic field." By
the time an electron has traveled a whole kiloparsec, it isn't
so 'high energy' any more.

High-energy
electrons are therefore local. Some members of the research
team believe the source could be less than a few hundred parsecs
away. For comparison, the disk of the spiral Milky Way galaxy
is about thirty thousand parsecs wide. (One parsec
approximately equals three light years.)

"Unfortunately,"
says Wefel, "we can't pinpoint the source in the sky."
Although ATIC does measure the direction of incoming particles,
it's difficult to translate those arrival angles into celestial
coordinates. For one thing, the detector was in the basket
of a balloon bobbing around the South Pole in a turbulent
vortex of high-altitude winds; that makes pointing tricky.
Moreover, the incoming electrons have had their directions
scrambled to some degree by galactic magnetic fields. "The
best ATIC could hope to do is measure a general anisotropy—one
side of the sky versus the other."

Right:
The ATIC cosmic ray detector ascends to the stratosphere tethered
to a high-altitude research balloon. More launch images:

This
uncertainty gives free rein to the imagination. The least
exotic possibilities include, e.g., a nearby pulsar, a 'microquasar'
or a stellar-mass black hole—all are capable of accelerating
electrons to these energies. It is possible that such a source
lurks undetected not far away. NASA's recently-launched Fermi
Gamma-ray Space Telescope is only just beginning to survey
the sky with sufficient sensitivity to reveal some of these
objects.

An
even more tantalizing possibility is dark matter.

There
is a class of physical theories called "Kaluza-Klein
theories" which seek to reconcile gravity with other
fundamental forces by positing extra dimensions. In addition
to the familiar 3D of human experience, there could be as
many as eight more dimensions woven into the space around
us. A popular yet unproven explanation for dark matter is
that dark matter particles inhabit the extra dimensions. We
feel their presence via the force of gravity, but do not sense
them in any other way.

How
does this produce excess cosmic rays? Kaluza-Klein particles
have the curious property (one of many) that they are their
own anti-particle. When two collide, they annihilate one another,
producing a spray of high-energy photons and electrons. The
electrons are not lost in hidden dimensions, however, they
materialize in the 3-dimensions of the real world where ATIC
can detect them as "cosmic rays."

"Our
data could be explained by a cloud or clump of dark matter
in the neighborhood of the solar system," says Wefel.
"In particular, there is a hypothesized Kaluza-Klein
particle with a mass near 620 GeV which, when annihilated,
should produce electrons with the same spectrum of energies
we observed."

Testing
this possibility is nontrivial because dark matter is so,
well, dark. But it may be possible to find the cloud by looking
for other annihilation products, such as gamma-rays. Again,
the Fermi Space Telescope may have the best chance of pinpointing
the source.

"Whatever
it is," says Adams, "it's going to be amazing."

For
more information about this research, see "An excess
of cosmic ray electrons at energies of 300-800 Gev,"
by J. Chang et al. in the Nov. 20, 2008, issue of Nature.

Credits:
The Advanced
Thin Ionization Calorimeter is an international
collaboration of researchers from Louisiana State University,
University of Maryland, Marshall Space Flight Center,
Purple Mountain Observatory in China, Moscow State University
in Russia and Max-Planck Institute for Solar System
Research in Germany. ATIC is supported in the United
States by NASA and flights are conducted under the auspices
of the Balloon Program Office at Wallops Flight Facility
by the staff of the Columbia Scientific Balloon Facility.
Antarctic logistics are provided by the National Science
Foundation and its contractor Raytheon Polar Services
Corporation.